This invention is generally related to micro-electromechanical switches, and more particularly to a structure and method for minimizing stiction between metal-to-metal contacts.
A wide variety of communications systems, such as mobile phone handsets, are known to require switches for directing the signal flow between the systems. An example is the need to switch a mobile phone antenna between the phone transmitting and receiving blocks. Suitable switches must allow the signal to pass through with low loss in the on state (low insertion loss) and provide good isolation between terminals in the off state.
Micro-electromechanical switches have become an increasingly attractive option for radio frequency (RF) switching because of their potential for low insertion loss and high isolation. In one class of micro-electromechanical switches, one contact consisting of a conducting film, is made to move or deflect to come into contact with another and close the circuit. The contacts are then separated again to open the switch.
A significant problem plaguing present art micro-electromechanical contact switches is the fact that the electrodes tend to stick to one another upon contact, making it difficult to separate them in order to turn the switch off. This phenomenon, known as xe2x80x9cstictionxe2x80x9d, is caused by the attraction at the microscopic level between atoms and molecules on the two surfaces. One solution is to ensure that when one contact plate is deflected to close the switch, the deflection creates a spring-like restorative force that naturally attempts to separate the contacts. If large enough, such a force can overcome stiction. However, the same force also implies that a large force must be generated to deflect the contact to close the device. In a switch wherein the deflection is electrostatic, this generally implies the need for a high control voltage beyond the 5V maximum that is required in, for instance, mobile handsets.
The stiction problem is not novel, and certain aspects of it have been described in the art. By way of example, in U.S. Pat. No. 5,772,902 to Reed et al., there is described a method for preventing adhesion of micro-electromechanical structures sticking to each other during fabrication. The structure described therein applies to micro-electromechanical systems but not to stiction that occurs during the operation of the switches. More particularly, the patent describes a method for shaping parts to avoid stiction when the part is fabricated and released.
Other solutions to modifying the restoring force of a micro-electromechanical switch have been described in the patent literature as, for instance, in U.S. Pat. No. 5,901,939 to Cabuz et al., wherein the use of multiple control electrodes and a specially shaped beam to create a stronger restoring force are described. The technique described, however, requires driving multiple electrode pairs in a two-phase configuration, which adds to the cost and complexity of the system employing such a switch. In addition, rather than using a deflecting beam, this switch relies on shifting a buckled region of a metal line toward one end of the line or the other, a technique which generates large flexure of the line and which can generate long-term reliability concerns.
The problem created by stiction during a transition of the switch from the on to the off state has been mainly addressed by investigating such a behavior when it occurs only during the manufacturing process and not during post-production switch operation. Further, most solutions fail in that they do not provide a continued use of a simple, single control electrode or multiple electrodes, all of which are actuated with voltages that are approximately in phase. Moreover, existing solutions fail to introduce an additional restoring force to the deflecting beam by means of a simple electrode coating rather than by employing a new type of beam that is difficult to manufacture and which is normally energized by buckling rather than by deflection (thus introducing high material stresses which have reliability implications).
Accordingly, it is an object of the present invention to provide a micro-electromechanical switch having a restoring force sufficiently large to overcome stiction,
It is another object to provide the micro-electromechanical switch with multiple electrodes coated with an elastically deformable film or with a layer of mechanical, conductive, deformable elements.
It is still a further object to provide the micro-electromechanical switch with a restoring force which is initially governed by a single spring constant k0 upon first application of a control voltage, and which is then supplemented by additional spring constants k1 through kn once the switch is near closure.
It is yet another object to provide a micro-electromechanical switch wherein contact is made gradually instead of an entire planar surface coming into contact and having to break the contact at once.
It is still a further object to provide a micro-electromechanical switch such that a contact plate or beam is deflected to close the switch, the deflection creating a spring-like restorative force that naturally separates the contacts.
In addressing the above objects, the present invention discloses a switch that is energized by way of a low control voltage despite the stiction phenomenon.
The invention builds upon an established micro-electromechanical switching concept consisting of a deformable beam which is anchored at least on one end. A voltage between the beam and the control electrode causes the beam to bend, coming closer to the control electrode. At a sufficiently large voltage, the deflection enables the beam to contact a second electrode, completing the electric circuit and closing the switch. The control electrode is preferably coated with a non-conducting material or placed slightly below the plane of the switch electrode in order to avoid contacting the beam itself during switch closure.
In another aspect of the invention, the micro-electromechanical switch is provided with a deflecting beam designed to come into contact with one or more contacts (or switch electrodes) having a compressible coating which may be either a continuous film, possibly with a non-smooth contacting surface, or a collection of discrete spring-like elements. The multiple electrodes provided with the coating are elastically deformable and are provided with a surface which may be rough in its uncompressed mode but which becomes flat when compressed. In addition to making contact gradually rather than having an entire surface coming into contact and having to break that contact at once, there is an added elastic energy introducing a non-linear increase in the separating force as the switch comes near its closed position, which also helps to reduce the problem of stiction.
The present invention begins with a relatively well-studied structure, i.e., a deflecting beam switch. Such a switch is nominally governed by a single spring constant, k0. This invention adds at least one additional constant, k1, which is activated only once the switch is near closure. Thus, by adding a conductive, compressible element to the switch electrode, the switch begins to close using only a low control voltage to overcome the spring constant k0. Near closure, when k1 is activated, the restoring force of the switch increases. This permits the switch to overcome stiction and, thus, to open more readily. Since k1 is not activated until the switch is almost closedxe2x80x94at which point the gap between beam and control electrode is smallxe2x80x94the required control voltage for completing switch closure remains low. This characteristic is highly desired in many modern applications for such switches, such as battery-powered cell phones.
Since the deflecting beam micro-electromechanical switch is governed by the main spring constant, k0, namely, that of the deflecting beam, it is not possible to facilitate opening the switch (i.e., ease contact stiction by increasing the spring constant) without also making it, at the same time, more difficult to close the switch. The consequence of this fact is a requirement for a large control voltage, which is contrary to the need of typical micro-mechanical switch applications, such as portable cellular phones operating from 3.6 to 7.2 volt batteries.
By introducing at least one additional spring constant, k1, the invention adds a new degree of freedom to the switch design. Upon initial application of the control voltage, the switch begins to close in a typical deflecting beam fashion, with the actuating force overcoming the spring constant k0. This constant is designed to be relatively low so as to allow a low control voltage. Part way through closure, however, spring constant k1 is turned on. A preferred mechanism is for the beam to contact a compressible, conductive coating on the switch electrode, causing it to compress. Spring constant k1 allows for a strong restorative force, helping the switch to overcome contact stiction and, thus, to open properly. However, k1 is not activated until the switch is almost fully closed and the gap between beam and control electrode is, thus, small. As a result, a low control voltage is sufficient to generate the force needed to overcome the sum of k0 and k1 and to allow proper closure of the switch.
In yet another aspect of the invention, the switch may also be implemented with multiple compressible layers stacked one on top of the other on the switch electrode, forming a layered coating and introducing additional spring constants k1, k2 . . . , kn, where n represents the number of stacked layers. Such an arrangement allows the overall curve representing the restoring force as a function of beam deflection to be more precisely tailored compared with the case of a single compressible layer.
Further, the present invention may replace the compressible layer with a series of metal spring-like structures which are anchored and in electrical contact with the switch electrode on one end and which protrude into the gap above the switch electrode. As the beam is deflected by the control voltage with spring constant k0, it comes into contact with the deformable, spring-like elements, causing them to compress and introducing the additional spring constant k1. The principle of operation remains the same as in the case of the compressible film, but with the additional spring constant introduced by a mechanical spring-like structure rather than by a compressible film.